fix(css): remove ?used hack (fixes #6421, #8245)

[sapphi-red]

Description

This ?used hack was introduced by 3e3c203.

At that time, esbuild v0.12.25 was used. This version of esbuild had trouble with tree-shaking.
This is the reproduction with esbuild repl. You can see linked variable and imported variable exists in the output.

But now Vite uses esbuild v0.14.38 and tree-shake works. This is the reproduction with esbuild repl. You can see linked is removed from output. imported is still included because of new URL('---', import.meta.url).href.
So here it needs to be marked as pure.
esbuild 0.13.0+ supports /* #__PURE__ */ and it worked if I marked by using it (esbuild repl).
The same code worked successfully with terser too.

This PR removes the ?used hack and changes the css default export to export default /* #__PURE__ */ (() => ${JSON.stringify(content)})() which I showed above.
I also tested with this project (stackblitz).

commit	index.js size	tree-shake	notes
67743a3	872B	⭕	the parent commit of this PR
a49bd36	13984B	❌	because I removed ?used hack
197253e	872B	⭕	because I added pure comment

fixes #6421
fixes #8245

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

This is too awesome 🔥

[sapphi-red]

This PR makes the output of import content from 'foo.css' to

const content = (() => "body{color:red}")()
console.log(content)

.

But this might hurt runtime perf if import content from 'foo.css' is used many places. https://v8.dev/blog/preparser#pife

const content = (function () { return "body{color:red}"})()
console.log(content)

This might be better for perf though the code is longer.

I'm not making a PR for this as I didn't measure the perf and it is an edge case but still leaving a note here.